Used to see this sort of thing all too often. The navy would use phosphorescent die in a mist from a drop capsule to help them study wind currents. Looked like the blob on helium. It would casually drift away and disperse over time.

Not saying that's what it was, but if you want a real story,

Back in about '79 I was living on Bolling Air Force Base (across the Potomac from D.C. The Naval Observatory just down the road reported to the local news stations that they were observing a UFO hovering above DC. We all went outside and observed a small object hovering in the sky which seemed to be rotating. It would turn from green to white to red and back to green again. A short while later all of the TV stations went off the air (not white noise and static either but slate gray like the TV was turned off even though the power was still on). After about an hour and a half the object slowly started receeding from our vantage point. It then suddenly zoomed up into the stratosphere and disappeared. The TVs came back on. The next day my mother called the observatory to ask if they had identified the object. When asked if she was a member of the observatory she said no instead of yes and was hung up on. True story.